Stretching and Squeezing the Bits

If you're working with images in Photoshop, they're probably made up of one or more 8-bit channels, in which each pixel is represented by a value from 0 (black) to 255 (white). Grayscale images have one such channel, while color images have three (RGB or Lab) or four (CMYK). If you're more adventurous, you may work with high-bit images, where each channel uses 16 bits per pixel to represent a value from 0 (black) to 32,768 (white).

The key thing to bear in mind is that when you use Photoshop's tonal controls, you're stretching and squeezing various parts of the tonal range, and in doing so, you inevitably lose some information. A key point to understand is that you lose a great deal more information in 8-bit-per-channel files than you do in high-bit ones. To demonstrate this, try the following simple experiment.